1. Field of the Invention
Embodiments of the invention generally relate to optical sensor systems. More particularly, embodiments of the invention relate to an optical wavelength interrogator to be used for interrogating fiber Bragg grating (FBG) sensors.
2. Description of the Related Art
Fiber Bragg gratings (FBGs), through measurements of Bragg wavelengths, can be used to detect any perturbations, such as temperature or strain (at the locations of the FBGs), which change the physical period of the refractive index modulation and/or the effective refractive index seen by the propagating light along the FBG, and hence the Bragg wavelength. These FBG sensors can be multiplexed along one or several fibers by writing the FBGs at different wavelengths that do not overlap under sensor operation.
One or several reflected FBG sensor wavelengths can be measured using a broadband source provided the source spectrum covers all possible FBG sensor wavelengths. These techniques for measuring FBG sensor wavelengths using a broadband source enable simultaneous wavelength demultiplexing and demodulation (wavelength determination) of the various FBG sensors. The transmission wavelength of the tuneable filter (or the laser source) will normally be scanned over the complete wavelength range of the sensors, where the control voltages to the tuning element, or the scan times, corresponding to maxima in the detected power are measures of the sensor Bragg wavelengths. The relationship between the control voltage, or scan time, and the tuning wavelength, i.e., the filter response, will for practical tuning elements not be linear, and will suffer from drift and hysteresis in the filter response. This is particularly true for a PZT-driven tuning element. For these reasons, some sort of reference scheme is required to measure Bragg wavelengths with high accuracy and repeatability.
One method for providing the reference scheme includes using a reference grating of known Bragg wavelengths arranged at the start of each of the series of FBGs. However, this means that the wavelength band of the reference FBG of each interrogator is dedicated to the reference FBG and is not available for use by a sensor FBG. Accordingly, use of the reference grating at the start of each series of FBGs limits and restricts the available optical bandwidth for the sensor FBGs.
A reference element in the form of a gas absorption cell has been shown in U.S. Pat. No. 6,421,120 (“the '120 patent”). The '120 patent describes an optical wavelength apparatus with a wide wavelength range which is illuminated by a wideband source. Suitable secondary devices including etalons, such as Fabry-Perot filters and Mach-Zehnder interferometers, are also described as wavelength reference elements. An absorption line in the gas absorption cell is used as a transfer standard to calibrate the response of a secondary reference over the range of a first reference.
Further, U.S. Pat. No. 6,587,484 (“the '484 patent”) describes a method and apparatus for determining a transmission wavelength for lasers in a dense wavelength division multiplexer. The apparatus of the '484 patent includes both a gas reference cell and an etalon being used to calibrate a transmission laser in a dense wavelength division multiplexer (DWDM) system.
In FBG sensing networks the amplitudes of the signals reflected from the FBG sensors may differ significantly between each sensor. The dynamic range of the receiver may then in many situations be too low to be able to measure the FBG sensors having small amplitudes of the corresponding reflection signals at the detection end (i.e., large losses). Hence, it is not possible to choose an optimum receiver sensitivity which covers all sensors. A receiver sensitivity sufficiently high to measure reflected signals of low amplitude can saturate the detectors for strong signal reflections from the FBGs.
Therefore, a need exists to have an FBG sensing network with a higher total dynamic range at the receiver end. A further need exists for an FBG sensing network with improved optical bandwidth for the sensor FBGs.